An Afghan man cultivates poppy bulbs at a farm in early May, 2011 near the city of Kandahar, Afghanistan. Myanmar is second to Afghanistan in opium production, according to a UN report released Oct. 31, 2012.

The United States military has blacklisted Afghanistan's leading private airline for allegedly ferrying large quantities of opium on civilian flights, the Wall Street Journal reported.

According to military officials interviewed by the WSJ, Kam Air smuggles the drug to Tajikistan, where it is distributed to the rest of the world.

Kam Air, the WSJ said, denies the charges. It's not clear how much business the airline may lose as a result of the blacklisting; Kam Air is the first major Afghan company to be penalized by the US military over drug allegations, the WSJ said.

"Kam Air is too large of a company not to know what has been going on within its organization," a US Army commander said to the WSJ.

A United Nations report in November found that while opium harvesting was down by a third in Afghanistan, cultivation of poppies was up 18 percent. According to UN officials, as long as Afghan farmers are committed to growing poppies, Afghanistan will remain a major producer of opium and heroin.

The Afghanistan government has been ambivalent about cutting down on the opium trade. Meanwhile, the Taliban and other militant groups often tax poppies in the areas under their control. According to The New York Times, the Taliban probably made at least $155 million from poppies in 2012.

Tajikistan's government also declined to comment on the allegation, according to the WSJ. Tajikistan is the poorest former Soviet country, and according to the WSJ, it's likely to become a major trafficking route for Afghan opium to reach the rest of the world. Many drug smugglers there enjoy high-level government protection.